The electric power steering device (EPS) for a vehicle assists the steering effort of the vehicle operator. The steering torque applied to the steering wheel causes an input shaft to rotate, and is transmitted to an output shaft. The steering torque transmitted to the output shaft is detected by a torque detecting device, and the steering torque of the output shaft is assisted as required according to the detected steering torque. The output shaft is typically affixed with a worm wheel which forms a worm gear mechanism in cooperation with a worm connected to an electric motor and meshing with the worm wheel. The EPS activates the electric motor in dependence on the detected steering torque, and this driving torque is transmitted to the output shaft via the worm and the worm wheel to be added to the steering torque.
The worm wheel is typically formed as a composite structure including a metallic core member affixed to the output shaft and a plastic gear forming member attached to the peripheral part of the core member and provided with teeth that mesh with the worm. The metallic core member ensures the worm wheel to be securely attached to the output shaft, and the plastic gear forming member reduces the noises and vibrations caused by the meshing of the gear teeth with the worm. See Patent Document 1.